Your Burden, My Burden?

The Ninth Circuit just came down with an interesting opinion reversing the conviction of a defendant for illegal reentry under s. 1326(a). 

The government convinced the trial judge to find that the defendant's claim that although he was born outside of the USA, he had derivative citizenship, was an affirmative defense.  Not so fast says Reinhardt - it is the government's burden to prove the defendant is an illegal alien, not the defendant's burden to prove he is not.

US v. Sandoval-Gonzalez

Curious if anybody has run across this issue in this district.

Why is it that federal prosecutors are constantly doing everything they can to prevent defendants from presenting a defense?  How does it harm the government if a defendant calls into doubt an element of its case?